(a) Field of the Invention
The present invention relates to a 180xc2x0 phase shift circuit having an improved isolation characteristic and, more particularly, to a 180xc2x0 phase shift circuit having an improved isolation characteristic as well as phase shift characteristics.
(b) Description of the Related Art
A 180xc2x0 phase shift circuit is a balance-unbalance converter which converts an unbalanced signal, input between an unbalance terminal and the ground, into a pair of balanced signals having an equal amplitude and a phase difference of 180xc2x0 therebetween by using a power distribution, and thus delivers the pair of balanced signals through a pair of balanced signal output terminals. The principal part of the 180xc2x0 phase shift circuit is called xe2x80x9cbalunxe2x80x9d in this technical field In the field of microwave circuits, 180xc2x0 phase shift circuits are widely used as a power distribution/synthesis circuit for a power amplifier, a balanced modem circuit, a mixer, and a phase shift device. A variety of proposals have been made for improving the characteristics of the 180xc2x0 phase shift circuit.
Patent Publication JP-A-7-131277 describes a 180xc2x0 phase shift circuit, such as shown in FIG. 1, which compensates degradations in an amplitude-difference characteristic and a phase-difference characteristic between the pair of balanced signals FIG. 1 shows an example of the 180xc2x0 phase shift circuit used for an amplifier block, wherein the 180xc2x0 phase shift circuits 51A and 51B are used as an input circuit and an output circuit, respectively, for the amplifier block 43 having a pair of balanced signal input terminals T21 and T31 and a pair of balanced signal output terminals T22 and T32.
Each of the 180xc2x0 phase shift circuits 51A and 51B is connected at the pair of balanced signal terminals T21 and T31 or T22 and T32 to the terminals of the amplifier block 43 after an impedance matching of the 180xc2x0 phase shift circuits 51 with the amplifier block 43. Thus, the amplifier block 43 receives input balanced signals from the 180xc2x0 phase shift circuit 51A to deliver output balanced signals to the 180xc2x0 phase shift circuit 51B. In this configuration, the 180xc2x0 phase shift circuit 51B delivers an output unbalanced signal having a reduced distortion within a wide band, which improves the characteristics of the amplifying system as a whole including the amplifier block 43 and the pair of 180xc2x0 phase shift circuits 51A and 51B.
Each of the 180xc2x0 phase shift circuits 51A and 51B includes a balun 41 and an amplitude/phase correction circuit 42. The balun 41 in the 180xc2x0 phase shift circuit 51A converts the unbalanced signal supplied through an input port P1 to deliver a pair of balanced signals through the output ports P2 and P3. The balun 41 in the 180xc2x0 phase shift circuit 51B converts balanced signals supplied through the input ports P2 and P3 to deliver an unbalanced signal through an output port P1 thereof. The amplitude/phase correction circuit 42, connected between the output port P2 and balanced signal output terminal T21, is implemented by a distributed parameter line having a specific characteristic impedance and a specific length for compensating or correcting the characteristics of the 180xc2x0 phase shift circuit for the amplitude difference and the phase difference between both the balanced signal.
In the 180xc2x0 phase shift circuit, if a reflected wave is generated due to an impedance mismatching on one of the pair of balanced signal terminals, the reflected wave is transferred though the 180xc2x0 phase shift circuit to the other of the pair of balanced signal terminals as a leakage signal. The leakage signal generates an adverse effect on the function of the circuitry unless the 180xc2x0 phase shift circuit has an excellent isolation characteristic. In the described circuitry, however, the isolation characteristic is not considered on the premise that a sufficient impedance matching is attained in the circuitry.
It is therefore an object of the present invention to provide a 180xc2x0 phase shift circuit having improved phase shift characteristics and an improved isolation characteristic.
The present invention provides a 180xc2x0 phase shift circuit including a first through third signal terminals, a balun including first and second ports for receiving/delivering a pair of balanced signals and a third port for delivering/receiving an unbalanced signal, a first impedance matching line connected between the first port and the first signal terminal, a second impedance matching line connected between the second port and the second signal terminal, and a serial branch including first and second resistors and a xcexg/2 distribution parameter line connected between the fist resistor and a second resistor, the serial branch being connected between a first node connecting together the first impedance matching line and the first signal terminal and a second node connecting together the second impedance matching line and the second signal terminal.
In accordance with the 180xc2x0 phase shift circuit of the present invention, reflected signal generated outside the 180xc2x0 phase shift circuit due to impedance-mismatching and entering the 180xc2x0 phase shift circuit through one of the balanced signal terminals cannot pass through the other of the balanced signal terminals. Thus, the isolation characteristic of the 180xc2x0 phase shift circuit can be improved without degrading the phase shift characteristics of the 180xc2x0 phase shift circuit.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.